One of the most outstanding problems in heterogeneous catalysis concerns its use in the partial oxidation of methane to form more reactive chemicals such as ethane, ethylene and other aliphatic hydrocarbons. The oxidative coupling of methane to form ethane and ethylene has been the subject of extensive research following the work of pioneers, Keller and Bhasin, in 1982. Particularly, the conversion of methane to ethylene has been widely investigated. Most researchers have been seeking to increase the efficiency of the conversion.
Much research has been performed and many patents have issued for the use of elements in Groups III, IV, V of periodic table of elements, known as the transition metals. Alkali oxide metals, earth metal oxides and even metal oxide complexes, have been used as catalysts for the conversion of methane to ethylene. Recently, a number of papers regarding methane conversion to ethylene have been published including one by this inventor entitled “Reactivity of ATiO3 perovskites Type of Catalyst (A=Ba, Sr, Ca) on Oxidative Coupling of Methane Reaction”, Research Institute of Petroleum Industries (RIPI) NIOC, Vol. 3, No. 10, 1993, in which well defined structures of this catalyst prepared by sol-gel methodologies are discussed. The characterization of these structures has been performed by X-ray diffraction (XRD), scanning electron microscope (S.E.M), the Brunauer, Emmett and Teller method (B.E.T) and confirmation of carbonate present at the surface as determined by Fourier transform Infrared spectroscopy (FTIR).
The kinetics and the mechanism of oxidative coupling of methane using a sodium-manganese oxide catalyst has been studied and confirms the mechanism for other similar structured catalysts. The results of this study was reported in a paper titled “Kinetics and mechanism of oxidative coupling of methane over sodium-manganese oxide catalyst” Rahmatolah et al., Chem Eng Technol 16(1993) 62-67. The reaction using the sodium-manganese oxide catalyst follows the Rideal-Redox mechanism, involving both homogeneous and heterogeneous reaction steps. Gas phase formation of a CH intermediate is a result of a heterogeneous process (surface reaction) and the formation of C2+ hydrocarbons by coupling methyl groups (CH3) is the result of a gas phase homogeneous process.
Catalytic activity for the conversion of methane to ethylene depends upon the surface oxidation rate constant (Kox) and the reduction rate constant (Red) between surface oxygen (O2) and methane (CH4). It was shown that the kinetic results of the Mn catalyst could be expressed by the rate equation based on the Rideal-Redox mechanism. Further, as reported in “Oxidative coupling of methane to ethylene over sodium promoted manganese oxide”, Golpasha et al., Journal of Engineering, Islamic Republic of Iran, Vol. 3, No.s 3 & 4, November 1990, it was found that the manganese oxide catalyst promoted with sodium and supported on silica exhibits fairly good activity and selectivity towards the synthesis of ethylene from methane at optimum operating conditions having a temperature of 830° C. at atmospheric pressure with a ratio of methane to oxygen of 2/1 (CH4/O2=2).
Further, the opportunity for commercialization of conversion of natural gas to ethylene through direct conversion reactions was studied in a paper presented by this inventor entitled “An opportunity for commercialization of natural gas to ethylene through direct conversion” at IIES-NIOC 16th World Petroleum Congress, June 2000, Calgary, Alberta, Canada. The paper compared existing processes for gas conversion from various points of view, including: capital requirements, economies of scale, and catalyst performance, and shows there is a possibility for commercialization of oxidation coupling of methane (OCM) over Pyrochlores type catalyst prepared using a sol-gel method. The experimental yield of hydrocarbon product (HC) from the reported catalyst reaction is 18-20% at 750-830° C. at atmospheric pressure over Sm2Sn2O7 catalyst (4).
One of these inventors has also reported, in a paper entitled “Direct Oxidative Methane Conversion To Ethylene Using Perovskite Catalyst, 14th World Petroleum Congress, 1994, Stavenger, Norway, the catalytic oxidative coupling of methane over a perovskite catalysts, CaTiO3, prepared using a modified ceramic method resulting in a HC yield of over 18-20% at 830° C. The catalyst was promoted with Na4P2O7, which did not considerably improve catalyst performance.
Other technology includes U.S. Pat. No. 4,939,310 which discloses a method for converting methane to a higher HC product using manganese oxide at 500°-1000° C. in contact with the mixture of methane and oxygen. U.S. Pat. No. 4,443,649 discloses a method for conversion of Methane to C, using the same catalyst as U.S. Pat. No. 4,939,310, but further comparisons using Ni, Rh, Pd, Ag, Os, Ir, Pt and Au instead of manganese were carried out. U.S. Pat. Nos. 4,443,649 and 4,544,787 show the use of manganese in different sorts of catalyst. U.S. Pat. No. 5,695,618 teaches the oxidative coupling of methane using an octahedral molecular sieve as the catalyst. U.S. Pat. No. 6,096,934 teaches methane conversion to ethane and ethylene using the same catalyst as U.S. Pat. No. 5,695,618, using steam in the same process. U.S. Pat. No. 5,877,387 utilizes a Pb-substituted hydroxyapatite catalyst for the oxidative coupling of methane which will occur at 600° C. U.S. Pat. No. 4,523,050 for conversion of methane report higher HC. In this patent, a methane and oxygen mixture is contacted with the surface of a solid catalyst containing Si, Mn or manganese silicate. Similarly, in U.S. Pat. No. 4,523,049, methane conversion using a manganese catalyst is promoted using an alkali or alkaline earth metal. U.S. Pat. No. 4,544,784 teaches the conversion of methane into hydrocarbon using catalysts of metal oxides of manganese which incorporate halogen compounds. This strengthened catalyst promotes more efficient conversion and greater contact of the gas mixture with the solid surface of the catalyst. U.S. Pat. No. 5,051,390 teaches the preparation of a cogel catalyst in an aquatic solution. Soluble salts of alkali metals and alkaline earth metals are mixed with soluble metals which are thermally decomposable to form a metal oxide and a hydrolysable silane under such conditions that a homogenous cogel is formed avoiding the formation of precipitates and particles. The catalyst produced is then used for conversion of methane into heavier hydrocarbons like ethylene and ethane.
To date, catalysts prepared using conventional techniques and a variety of constituents are only capable of converting methane to higher hydrocarbons such as ethylene at an efficiency of approximately 20%. Ideally, catalysts for the direct conversion of methane to higher hydrocarbons would be capable of producing said higher hydrocarbons at an efficiency of greater than 20% to provide a commercially viable method for the production of ethylene.
The present inventors have also identified a way to maintain the activity of the catalyst over time, by periodic addition of a halogen source to the OCM reactor over the course of the OCM conversion.